They're designed so the frame or lens doesn't touch your face if you have an "asian" face. Different bone structures and all that. Most of the frames just have different nose bombs or the nose bombs flip around to go from asian fit to regular, some frames are different altogether to accommodate.

On most of the Sport frames, the Asian Fit has a flatter curve and wider opening between the earstems, and the larger nose pads. The Flak Jacket is a good example of this. The flatter curve of the frame as well as the wider opening on the earstems allows a more comfortable fit on wider heads and flatter faces on Asians. On the Lifestyle frames, it's almost always only a larger nosepad. ie Valve, Scalpel, Two Face, Frogskins. The larger nosepads hold the frames further away from the face so eyelashes and eyelids don't rub on the inside of the lenses on flatter faced Asian faces with small or no nosebridge.
There are of course many exceptions to both of these general descriptions.
I am Asian and some Asian Fits with different nosepads only, still do not fit my big, wide head and flat face, and small nosebridge. The only thing you can do is try on both types to see what fits your face better.